This is a request for a Senior Scientist Award. The long-term objective is to understand the neural basis of motivational states. Disorders of motivational states are associated with a wide variety of mental and psychological diseases, but the neurobiology of these states is poorly understood. Feeding behavior in the mollusc Aplysia has been used extensively as a model system to gain insights into the principles that can provide insights across different species including humans. Motivational systems appear to function, so as to optimize behavior and both intrinsic and extrinsic modulatory neuronal systems perform this function. A variety of modulatory mechanisms act both centrally and peripherally to adjust the parameters of the system, so as to tune them to operate efficiently under different conditions. It is now proposed to extend the use of the Aplysia feeding system to gain insights into how motivational systems function to determine behavioral choice. It is suggested that choices are determined either by using built in computations based on existing wiring and software or by altering the properties of the components and connections by means of neuromodulators. We will explore the hypothesis that the nervous system will use combinations of these strategies and the type of mechanisms will be a function of the types of behaviors involved and perhaps the evolutionary history underlying the order in which the different response systems evolved. The research will use a combination of cellular, molecular and electrophysiological techniques, combined with several computational approaches.